Many homes and other buildings rely upon furnaces to provide heat during cool and/or cold weather. Typically, a furnace employs a burner that burns a fuel such as natural gas, propane, oil or the like, and provides heated combustion gases to the interior of a heat exchanger. The combustion gases typically proceed through the heat exchanger, are collected by a collector box, and then are exhausted outside of the building via a vent or the like. In some cases, a combustion blower is provided to pull combustion air into the burner, pull the combustion gases through the heat exchanger into the collector box, and to push the combustion gases out the vent. To heat the building, a circulating air blower typically forces return air from the building, and in some cases ventilation air from outside of the building, over or through the heat exchanger, thereby heating the air. The heated air is then typically routed throughout the building via a duct system. A return duct system is typically employed to return air from the building to the furnace to be re-heated and then re-circulated.
In order to provide improved fuel efficiency and/or occupant comfort, some furnaces may be considered as having two or more stages, i.e., they have two or more separate heating stages, or they can effectively operate at two or more different burner firing rates, depending on how much heat is needed within the building. Some furnaces are known as modulating furnaces, because they can operate at a number of different firing rates. The firing rate of such furnaces typically dictates the amount of gas and combustion air that is required by the burner. The amount of gas delivered to the burner is typically controlled by a variable gas valve, and the amount to combustion air is often controlled by a combustion blower. To obtain a desired fuel to air ratio for efficient operation of the furnace, the gas valve and the combustion blower speed are typically operate in concert with one another, and in accordance with the desired firing rate of the furnace.
In some cases, when the firing rate is reduced during operation of the furnace, the flame in the furnace can be extinguished. In some cases, the safety features of the furnace itself may extinguish the flame. For example, a dirty flame rod, which may not be able to detect the flame at reduced firing rates, may cause a safety controller of the furnace to extinguish the flame. Likewise, ice buildup or other blockage of the exhaust flue, or even heavy wind condition, may prevent sufficient combustion airflow to be detected, which can cause a safety controller of the furnace to extinguish the flame, particularly at lower firing rates. If the flame goes out, many furnaces will simply return to the burner ignition cycle, and repeat. However, after ignition, the furnace may attempt to return to the lower firing rate, and the flame may again go out. This cycle may continue, sometimes without providing significant heat to the building and/or satisfying a current call for heat. This can lead to occupant discomfort, and in some cases, the freezing of pipes or like in the building, both of which are undesirable.